Magnetic focusing of cathode ray tubes



Oct. 22, 1940. D. J. MYNALL 2,219,194

MAGNETIC FOCUSING OF CATHODE RAY TUBES Filed April 21, 1938 Fig. I.

Invento r": Dennis J. Mynall,

i Atto ney.

Patented Oct. '22, 1940 MAGNETIO FOCUSING F GATHODE R Y TUBES Dennis James Mynall, Rugby, Eng-land, ;assignor to General Electric Company, .acorporation of New York Application April 21, 1938, Serial Norzogglz In Great Britain Ma 11, 1937 3 Claims. (chasm-'27) This invention relates to arrangements for the supply of current to magnetically focused cathode rayitubes, such as are usedin television reception, or for other purposes of (cathoderay Oscilloscopes).

The object of this invention is to provide a means for ensuring the constancy of focus of a magnetically focused cathode rayrtube despite variations of the Voltage of the electrical mains from which the lens-and tube derive their ,supplies. The present invention'is directly applicable to use with lenses of conventional construction...

- If I 'amperes is thewcurrentiin'the windings of one-half the percentage variation of -V in order tomaintain focus, whatever the initial value of V. :Now the type of IV relationship which is normally obtained in .practice is closely repre-'-- 'sentable' over the working range byI=bV+c, -where"b.and c are constants and c may be' zero but .is, in any case, smallv compared withxbv." It

:follows immediately that the practical relation- .ship givesa. percentage variation of I whichis 1/(1+c/bV) times that of V, and this factor does .not. depart appreciably from unity since c/bV maybe zero but is, in any case, small compared with unity. So that the actual change in I consequent upon a variation of V is twice the amount required to maintain constancy of focus, and it may be necessary to regain focus by manual adjustment whenever there is a change of mains voltage. This effect is undesirable in a television receiver.

The present invention comprises a circuit arrangement for supplying current to the magnetic lens in such a fashion that a change of I consequent upon a. change of mains voltage is automatically restricted to the amount required to compensate for the concomitant change of V, over an adequate working range.

The accompanying drawing shows the manner in which the invention is carried into effect.

' Fig. 1 is a circuit diagram illustrating the use of the invention in connection with a direct curlike nature rent source; Fig. 2 shows in somewhat greater detail the manner of useof the circuit='of:Fig. 1 with a cathode ray tube of conventionaliconstructi'o'n, andFigQS shows the applicationof the invention in a-case' where the primary 'voltage source is of the alternating current variety.

Fig. 1 shows the essentials of the invention; l is the lens winding having a resistance R1 ohms, 2' and '3 are resistors R2, and Raohms respec tively: 4 is a gas discharge voltage stabilising tube and 5 is a source of- D. C. voltage fed from the same supply mains as the cathode ray tube. Let E volts be thevoltage of the source 5 and e volts (e less than E) be the Working voltage of the stabilising tube 4. Then it is found that, if R2 and R3 are suitably proportionedwith regard to E and e, it is possible" to make the percentage change of I any desired fraction, between zero and unity, of the'p'erc'entage change of E. In particul'ar the percentage change of I may be made equal to one-half of thepercentage change of E, i} e. either exactly 'or very approximately 'one-' half of the percentage change of V, and'this is the condition requiredto maintain Ifocus. Explicitly, 6R3 must be made equal to ERz and when this relation holds R3 must be less than '(Eae) /I in order that the stabilising tube may function. The above conditions are not'sufiicient to determine the circuit elements absolutely and it is generally found convenient tochoose the mean current to be taken by the tube 4, when the circuit becomes determined and the values of R1,

R2, and R3 are readily calculable byfknownmethods.

For practical reasonsit may be desirable to make the resistors 2 and/or 3 variable elements in order to provide initial adjustment, and the current in the lens I may be initially adjusted by series and/or parallel resistance, in which case R1 must be taken to represent the resistance of the lens branch considered as a unit and I the I In Fig. 2 I have shown the application of the J circuit arrangement of Fig. l to a cathode ray tube of conventional type. The tube itself comprises a bulbous screen-enclosing portion l2 and an elongated shaft portion I3 which contains'a cathode I4, a modulating electrode l5 and an accelerating anode I6. The tube may include additional elements such, for example, as a system of deflecting electrodes (not shown).

Voltage is impressed between the cathode and anode by means of a direct current source conventionally represented as a generator 5. The focusing coil l, which is positioned around the shaft portion of the envelope, is energized by connection to this same source through a regulating circuit of the type described in connection with Fig. 1.

Fig. 3 shows the preferred embodiment of the invention. The elements I, 2, 3 and 4 have the same meanings as in Fig. 1. 6 is a variable resistor, the primary purpose of which is to provide initial adjustment of focus. A subsidiary function of the resistor 6 may be to reduce the variation of R1 with change of temperature of the coil winding, but this is not considered an essential feature of the invention. I is a condenser the function of which is to reduce the ripple voltage from the rectifying tube 8 so that the tube 4 is maintained continuously in action, and 9 is a transformer connected to alternating current supply mains via terminals Ill and II. Further smoothing, effective on the branch containing 4 and/or the branch containing I, may be employed if necessary. The method of smoothing does. not affect the direct current action of the circuit provided only that the smoothing is effective enough, and so is not considered to be a feature of the invention.

It will be recognised by those skilled in the art that the resistance of the windings of the transformer 9 and the internal D. C. resistance of the rectifying tube 8 together provide an effective proportion of the value of R3, and may even be designed to constitute the whole of it, in which case the resistor 3 is not required as a separate entity. It will also be appreciated that the source voltage, E, used in calculating the circuit is not the actual open-circuit output voltage of the rectifier but rather the fictitious open-circuit voltage which is obtained by linear extrapolation to zero load from the working range on the rectifier load characteristic.

It is found in practice that the variations which may exist between individual tubes constituting the elements 4 and 8 are not usually sufficiently large to .call for readjustment of the ratio R2/R3 when a replacement is made, so that it is practicable to make the resistors 2 and 3 fixed in value, all necessary adjustment being made by means of resistor 6.

WhatI claim as new and desire to secure by Letters Patent of the United States is:

1. For use with a cathode ray tube of the type which comprises electrostatic means for producing and accelerating an electron beam and magnetic means for focusing the beam, the combination which includes a common source for supplying voltage to the beam-accelerating means and current to the beam-focusing means, and circuit means interposed between the source and the focusing means for causing percentage variations of the focusing current produced by changes in the potential level of the said source to correspond closely to one-half the similarly produced percentage variations of the accelerating voltage, whereby a substantially constant focusing effect is obtained.

2. For use with a cathode ray tube of the type.

which comprises electrostatic means for producing and accelerating an electron beam and magnetic means for focusing the beam, the circuit combination which includes a common source of potential of normal value E for supplying voltage to the beam-accelerating means and current to the beam-focusing means, a series resistance of value R3 interposed between the said source and focusing means, another resistance of value R2 and a discharge device having a constant breakdown voltage e connected serially with respect to one another and to the first resistance but in shunt to the focusing means, the proportioning of the resistances and the voltages being such that eR3=ER2, whereby a constant focusing action is obtained irrespective of variations in the potential of the supply source.

3. For use with a cathode ray tube of the type which comprises electrostatic means for producing and accelerating an electron beam and magnetic means for focusing the beam, the combination which includes a common source for supplying voltage to the beam-accelerating means and current to the beam-focusing means, a series resistance interposed between the said source and focusing means, and another resistance and a discharge device having a constant breakdown voltage connected serially with respect to one another and to the first resistance but in shunt to the focusing means, the said resistances being so proportioned with respect to the voltage of said source and with respect to the breakdown voltage of the said discharge device that percentage variations of the focusing current produced by changes in said source correspond closely to one-half the similarly produced percentage variations of the accelerating voltage, whereby a substantially constant focusing effect is obtained irrespective of changes in the voltage source.

DENNIS JAMES MYNALL. 

